Golf balls are typically categorized into two groups: Solid and wound. Solid golf balls commonly include a core encased by a cover. The core may be either single layered or have multiple layers. The cover may also be single or multi-layered. Sometimes, an intermediate layer is disposed between the core and the cover. On the other hand, wound golf balls are generally constructed from a liquid or solid center encased by tensioned elastomeric material and a cover.
Golf ball manufacturers seek to maximize golf player performance under varying weather-related or topographical conditions and in view of individual playing styles. Meanwhile, golf ball specifications set by the USGA (United States Golf Association) are often considered. The USGA requires an overall golf ball diameter of at least 1.68 inches (42.67 mm), an initial velocity of up to 250 ft/sec. +2%, golf ball weight of up to 1.620 ounces, and distance of up to 280 yards, +6%.
The prior art is comprised of various golf balls that have been designed to provide optimal playing characteristics. These characteristics are generally the initial velocity and spin of the golf ball, which can be optimized for various players. For instance, certain players prefer to play a ball that has a high spin rate for playability. Other players prefer to play a ball that has a low spin rate to maximize distance. However, these balls tend to be hard feeling and difficult to control around the greens.
The prior art is comprised of liquid filled golf balls. Wound golf balls have been made with liquid filled centers for many years. Both U.S. Pat. Nos. 1,568,513 and 1,904,012 are directed to wound golf balls with liquid filled centers. U.S. Pat. Nos. 5,150,906 and 5,480,155, are directed to a hollow spherical shell of a polymeric material which is filled with a liquid or unitary, non-cellular material that is a liquid when introduced into the shell. The shell is disclosed as being the outer cover or an inner layer with the outer cover formed to the external surface thereof. The shell varies in thickness from about 0.060 to 0.410 inches in thickness.
Other known attempts to mold layers around a solid center entail placing a preformed center between two blocks of core material in a spherical compression mold, and closing the mold. This is done in the manufacture of golf balls sold by Kamatari. This process, however, provides little control over the ultimate placement of the center within the golf ball core. Large variations in center eccentricities can result.
The prior art also provides for the manufacture of double cover golf balls. This is generally accomplished by injection molding a first and then a second cover layer around a core. This system, however, requires complex injection molds, usually with retractable pins within the mold to properly position the core.
Very small core centers would be desirable to achieve the commonly varying characteristics, including resilience, compression, flexural and/or inertial properties. Previous uses of small diameters were either limited to wound constructions or troubled with very poor concentricity. Additionally, the use of very small centers was not previously possible because the stresses exerted by the subsequent molding processes either distorted or totally displaced the center.
It is also desirable to improve adhesion between outer core parts which mate around the inner core center and improve golf ball durability by eliminating the large fracture plane which presently exists with conventional planar mating surfaces. Accordingly, there is a need for a golf ball and a method of making the golf ball which permits reliable and cost effective use of very small core centers with outer core mating parts meanwhile improving desired golf ball properties and characteristics.